Electrostatic forces have been used to move structures. Traditional electrostatic devices were constructed from laminated films cut from plastic or Mylar materials. A flexible electrode was attached to the film, and another electrode was affixed to a base structure. Electrically energizing the respective electrodes created an electrostatic force attracting the electrodes to each other or repelling them from each other. A representative example of these devices is found in U.S. Pat. No. 4,266,399. These devices work well for typical motive applications, but these devices cannot be constructed in dimensions suitable for miniaturized integrated circuits, biomedical applications, or MEMS structures.
Micromachined MEMS devices have also utilized electrostatic forces to move microstructures. Some MEMS electrostatic devices use relatively rigid cantilever members, as found in U.S. Pat. No. 5,578,976. Other MEMS devices disclose curved electrostatic actuators. However, some of these devices incorporate complex geometries using relatively difficult micro-fabrication techniques. U.S. Pat. Nos. 5,629,565 and 5,673,785 use dual micromechanical substrates to create their respective electrostatic devices. The devices described in U.S. Pat. Nos. 5,233,459 and 5,784,189 are formed by using numerous process steps. Complex operations are required to create corrugations in the flexible electrodes. In addition, U.S. Pat. No. 5,552,925 also discloses a curved electrostatic electrode. However, the electrode is constructed from two portions, a thinner flexible portion followed by a flat cantilever portion.
Several of the electrostatic MEMS devices include an air gap between the substrate surface and the electrostatic actuator. The electrostatic actuators generally include flexible, curled electrodes. Typically, the gap starts at the beginning of the electrostatic actuator where it separates from the substrate surface and increases continuously along the length of the air gap. The size of the air gap increases as the actuator curls further away from the substrate surface along its length. The air gap separation between the substrate electrode and actuator electrode affects the operating voltage required to move the actuator. The larger the air gap, the higher the voltage required operating the actuator. Further, due to manufacturing process and material variations, size and shape of the air gap can vary substantially from device to device, making operation erratic.
MEMS actuators using electrostatic force as means of moving, shaping or actuating a payload are integral part of many, if not most Micro-Electro-Mechanical Systems (MEMS). They have low power consumption and small size. These include parallel-plate actuator, cantilever actuator, torsional drive, comb drive, rotary motor, and scratch drive. Of these, only parallel-plate actuator generates strict vertical (out-of-plane) displacement. A schematic of the parallel-plate actuator is shown in FIG. 1. It comprises a movable electrode 10, a fixed electrode 20, spring 82 as hinges, a pair of pillars 30 on substrate 1. The movable electrode 10 is suspended by the spring hinges 82, which have a spring constant k, and is substantially parallel to the fixed electrode 20 with air gap go in between. A voltage Vin applied between the two electrodes gives rise to a force F and a displacement that can be calculated by the following equations:
                    F        =                                                            ɛ                ·                A                ·                                  V                  in                  2                                                            2                ⁢                                  g                  2                                                      ⁢                                                  ⁢            and            ⁢                                                  ⁢            g                    =                                    g              o                        -                                          ɛ                ·                A                ·                                  V                  in                  2                                                            2                ⁢                                  k                  ·                                      g                    2                                                                                                          EQ        .                                  ⁢        1            where g is the instantaneous air gap, ε is dielectric constant, A is area of the smaller electrode. Note that this is now a cubic equation for the gap. As we increase the voltage, the air gap decreases, with the amount of decrease growing as the air gap gets smaller. Thus there is positive feedback in this system, and at some critical voltage, the system goes unstable, and the air gap collapses to zero. This phenomenon is called pull-in. The air gap at which the pull-in occurs is given by gPI=⅔·go. That is, the movable electrode is pulled down by ⅓ of the original air gap. At this value of air gap, the pull-in voltage is
                              V          PI                =                                            8              ⁢                              k                ·                                  g                  o                  3                                                                    27              ⁢                              ɛ                ·                A                                                                        EQ        .                                  ⁢        2            
The parallel-plate electrostatic actuators normally operate in the analog regime, that is, the initial ⅓ of the air gap, prior to the pull-in. An analog voltage signal is applied between the two electrode plates to attain a displacement given by EQ. 1. It can be seen that the displacement is a non-linear function of the applied voltage. Such non-linear behaviors present a challenge to many MOEMS (Micro-Optical-Electro-Mechanical-System) applications requiring accurate positioning, including tunable Fabry-Perot filter and vertical cavity surface emitting laser (VCSEL), used for wavelength band selection in optical communications. The accuracy requirement can only be met with closed-loop control schemes. In addition, the tuning range of the displacement is limited to a few micrometers, which restricts MOEMS devices to the visible or near IR (Vis-NIR) applications. For mid-wavelength IR (MWIR) or LWIR applications, where the tuning range is larger than 2 micrometers, very high voltage (100-200 volts) must be used, which restricts the application. The voltage is too high and needs to be decreased.
There is still a need to develop improved MEMS devices and techniques for leveraging electrostatic forces and causing motion within microengineered devices. Electrostatic forces due to the electric field between electrical charges can generate relatively large forces given the small electrode separations in MEMS devices. Electrostatic devices operable with lower and less erratic operating voltages are needed. Advantageous new devices and applications could be created by leveraging the electrostatic forces in new MEMS structures.